1. Field of the Invention
The present invention relates to a multi-point ignition system for an internal combustion engine for an automobile engine.
2. Description of Related Art
Multi-point ignition systems for automobile engines have a plurality of ignition spark gaps in each cylinder. In order to arrange a plurality of peripheral ignition spark gaps along an inner wall of a combustion chamber of each cylinder at appropriate circumferential separations, a plurality of spark plugs may be supported by an annular supporting ring. Such an annular supporting ring is disposed between a cylinder block and a cylinder head. Simultaneous fuel ignition by providing a spark across each of the plurality of peripheral ignition spark gaps causes rapid fuel combustion so as to realize ideal "iso-volume" combustion, particularly for Otto cycle engines. Such an ignition system is known from, for instance, Japanese Unexamined Patent Publication No. 57-148,021.
It has recently been attempted to provide an ignition spark gap at the center of a combustion chamber in addition to a plurality of peripheral ignition spark gaps. In order to optimize fuel combustion, these ignition spark gaps are selectively used to ignite fuel in different ignition modes; such ignition modes may include a first ignition mode and a second ignition mode.
When an engine operates under lower loads, the first ignition mode, in which only the peripheral ignition spark gaps are used to ignite fuel, is selected so as to cause sufficient fuel combustion along the inner wall of the combustion chamber. This suppresses increases in hydrocarbon (HC) which are produced easily in a peripheral area of the combustion chamber. Additionally, flame expansion in an early stage of fuel combustion is suppressed. Suppressing flame expansion lowers a peak speed of fuel combustion and leads to provision of a uniform combustion ratio throughout the combustion chamber during the entire period of combustion. This, in turn, results in a decrease in emission of nitrogen oxides (NOx). On the other hand, when the engine operates under higher loads, the second ignition mode, in which all of the ignition spark gaps are used at once to ignite fuel, is selected. This second ignition mode prevents the occurrence of engine knocking.
When considering both the relative placement of intake and exhaust ports and spark plug service efficiency, it is difficult to arrange the peripheral ignition spark gaps at equal circumferential separations. Having peripheral ignition spark gaps at unequal circumferential separations may produce a delay in what is known as "flame fusion" near part of the combustion chamber between adjacent peripheral ignition spark gaps having a separation larger than those of other adjacent peripheral ignition spark gaps. Such a delay in flame fusion is undesirable from the standpoint of providing sufficiently decreased hydrocarbon (HC) and nitrogen oxide (NOx) emissions.